Monolayer materials, such as graphene and hexagonal boron nitride (h-BN), are materials with great potential for electronics and other future device architectures based on atomically thin (monolayer) sheets. Graphene is a single atomic layer of sp2-bonded carbon (C) atoms densely packed into the form of a two-dimensional honeycomb crystal lattice. Graphene has been shown to be a zero-bandgap material whose charge carriers behave as massless Dirac fermions. It has remarkably high room-temperature charge carrier mobility with individual carriers exhibiting long-range ballistic transport. In the form of a monolayer sheet, h-BN is a single atomic layer of alternating boron and nitrogen atoms. In the bulk, h-BN has a graphite-like structure, in which such planar BN sheets are stacked on top of each other in a regular fashion. The large band gap of h-BN (both in the bulk and as a monolayer sheet) make this material capable of emitting deep ultraviolet radiation, which can be particularly beneficial for incorporation into a variety of optoelectronic devices, especially used for, as an example, nanometer lithography and white light in LED's.
Monolayers of graphene and h-BN are an attractive pair of materials that can be integrated to form 2D hetero structures in individual monolayer membranes. They are isostructural, nearly lattice-matched, and isoelectronic. As will be shown below, h-BN attaches preferentially to existing graphene domains and does not produce any secondary nuclei during growth at high temperature, which is consistent with the preferential incorporation of boron and nitrogen at the graphene edge.
The different band structures between graphene and h-BN generate a potential for interesting functional properties arising from the integration of the two materials in a heterogeneous monolayer membrane. Some of the unusual electronic properties that have been predicted in connection with the interfaces between monolayer graphene and h-BN include the opening of a variable bandgap, magnetism, unique thermal transport properties, robust half-metallic behavior without applied electric fields, and interfacial electronic reconstructions analogous to those observed in oxide heterostructures. Some of these unusual properties result because monolayer graphene boron nitride heterostructures have the potential to overcome the limitations due to the vanishing energy-gap of graphene.
Despite the extraordinary potential associated with interfaces between monolayer graphene and h-BN, access to these properties depends on methods for adequately controlling the formation of the interfaces between the monolayer graphene and h-BN domains within a single atomic layer. Harnessing these properties in large scale practical applications requires the identification of growth protocols and processing conditions tailored to their unique physical and chemical properties. Beyond the synthesis of the individual constituents (graphene, h-BN) as monolayer membranes, the synthesis of heterostructured 2D membranes with well-defined interfaces presents unique challenges, and raises fundamental questions on materials integration, such as interface formation and reduction of intermixing along boundary interfaces.
Techniques for the synthesis of two-dimensional (2D) materials and their heterostructures on metal substrates have become increasingly well developed. For instance, one investigation indicated that few-layer hybrids on Cu provide evidence for separate graphene and boron nitride domains on the nanoscale. (See Ci, L.; Song, L.; Jin, C.; Jariwala, D.; Wu, D.; Li, Y.; Srivastava, A.; Wang, Z. F.; Storr, K.; Balicas, L.; Liu, F.; Ajayan, P. M. Nat Mater 2010, 9, (5), 430-435, incorporated by reference in its entirety.) However, this investigation did not address interface formation between graphene and boron-nitride domains. Nor did it offer any insight into the ways that a metal substrate can cause modifications to the phase behavior of graphene and boron nitride domains.
Many of the unique properties predicted for monolayer graphene/h-BN heterostructures depend on the ability to fabricate such heterostructures with atomically sharp boundaries between the two constituents, that is graphene being bonded directly to h-BN without an intermixed ternary C—B—N “alloy” phase in between. However, in principle, several mechanisms could lead to mixing or alloying between graphene and h-BN. For example, substitution of carbon by boron or nitrogen at the graphene edge, direct substitution of carbon for boron or nitrogen (or vice versa) inside an existing domain, or carbon incorporation during the growth h-BN domains could result in substantial intermixing along the monolayer graphene/h-BN interfaces within monolayer graphene boron nitride heterostructures. All of these effects would lead to non-abrupt boundaries, in which the pure phases (graphene, h-BN) are separated by an intermixed zone containing C, B, and N instead of the desired atomically sharp boundary. Such non-abrupt, non-atomically sharp interfaces between monolayer graphene and h-BN hinder exploitation of the extraordinary potential associated with these interfaces in monolayer graphene boron nitride hetero structures. Consequently, identifying the extent of intermixing and controlling the formation of monolayer graphene/h-BN interfaces that limits a possible intermixing so as to achieve atomically sharp interfaces remains a challenge and has not heretofore been known or understood.